The “load index” of a tire is a parameter well known to those skilled in the art for quantifying the maximum load that the tire is able to bear when mounted on a mounting rim and inflated to its service pressure. A load index of 100 corresponds to a maximum load of 800 kg.
The assembly formed by the bead and the radially inner part of the sidewall of a tire is one of the components of the tire the structure of which has a very marked impact on the endurance of the tire. It performs many roles. For example, it absorbs the tension of the carcass reinforcement and transmits the load to which the tire is subjected from the sidewall to the rim. It therefore guides the crown of the tire from the rim. The influence that it has on the road holding of the tire is considerable, especially when the tire is heavily loaded. In the case of tires for passenger vehicles with a high load index, all of these functions are usually performed by combining a double carcass reinforcement (comprising the bead wire and the turn-up of this double carcass reinforcement about the latter) and a “bead filler” made of rubber composition. The compromise between the rigidity that is to be achieved, particularly for guiding the crown, and the expected endurance generally results in the double carcass reinforcement having to follow a certain path and in the use of a bulky (tall and/or thick) and rigid bead filler. The downside of this geometry is the complexity of the manufacturing process and the cost of the tire. The stiffening action of the bead filler is predominant especially in the region distant from the bead and therefore requires a bead filler that is all the more bulky and, as a result, a manufacturing process that is complex.
The need to reduce the complexity of the manufacturing process and the cost of the tire have led the manufacturers to use a single carcass reinforcement, even for tires with a high load index. The need to maintain a relatively low rolling resistance has made them use less rigid rubber compositions in the bead. The lower rigidity of these rubber compositions is then compensated for by the use of a relatively thick bead filler, possibly combined with an outer strip which is itself thick.
The use of these semi-finished products does nonetheless present a problem in terms of manufacture, because it requires the reels from which these semi-finished products are supplied to be replaced more frequently.
In order to address this problem while at the same time maintaining the performance of the tires concerned, the applicant has conducted research aimed at finding a more sensible distribution of the thickness of the filler on the bead wire and of the outer strip associated therewith.